Polarized Light Emission from Micro-Pixel Displays and Methods of Fabrication Thereof

ABSTRACT

A method and apparatus for achieving selective polarization states of emitted visible or other light in a stacked multicolor emissive display device by utilizing nonpolar, semipolar or strained c-plane crystallographic planes of semiconductor materials for light emitting structures within an electronic emissive display device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/672,060 filed Aug. 8, 2017, which claims the benefit of U.S.Provisional Patent Application No. 62/429,033 filed Dec. 1, 2016, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to solid state light emitters such as LEDs andlaser diode structures. More specifically, the invention relates tosolid state light emitting structures fabricated from III-nitridematerials that take advantage of III-nitride material's uniquecrystallographic properties whereby polarized light is emitted from thelight emitting structures.

2. Prior Art

Solid state light emitters are at the forefront of today'scommercially-available electronic display systems. Many display systemsutilize the properties of light polarization in order to obtainincreased contrast ratios between “on” and “off” pixels on the displayelement. The use of unpolarized light requires that such display systemsincorporate multiple polarizers and polarizing optics, making themlarger, more complex, less energy efficient, and costly. The materialsystem of III-nitrides affords a way to crystallographically tailor thepolarization state of the emitted light from the III-nitride material byselecting crystallographic planes in the material that favor certainpolarization states. This, in turn, allows for a solid state lightemitting system to be fabricated with a minimum amount of light emissionengineering, inherently making the system more efficient in terms ofpower consumption and design.

Certain commercially available LEDs utilize a III-nitride materialsystem. The AlGaInN material system, with its large bandgap ranging fromthe deep UV to near-infrared emission wavelengths is an attractivesystem for electronic displays since its output covers the entirevisible electromagnetic spectrum. The majority of GaN LEDs used withexisting display technologies are grown on the polar (0001) c-plane ofwurtzite GaN. Non-basal planes of GaN however, afford distinctadvantages and different optical properties to the traditional c-planeGaN. Undesirably, due to the polarization-related electric fields insidethe multi-quantum wells for c-plane GaN, the quantum confined Starkeffect (QCSE) causes a lower energy recombination transition and acharacteristic blue shift in the peak emission wavelength is observed ascurrent density is increased (T. Takeuchi, S. Sota, M. Katsuragawa, M.Komori, H. Takeuchi, H. Amano, and I. Akasaki, “Quantum-confined Starkeffect due to piezoelectric fields in GaInN strained quantum wells,”Japanese Journal of Applied Physics 36, L382-L385 (1997)). Anotherundesirable consequence of c-plane growth is the efficiency droop thatoccurs at higher current densities (Y. C. Shen, G. O. Mueller, S.Watanabe, N. F. Gardner, A. Munkholm, and M. R. Krames, “Augerrecombination in InGaN measured by photoluminescence,” Applied PhysicsLetters, 91, 141101 (2007)).

One of the first high quality, nonpolar growth materials was reported onthe m-plane of GaN in 2000 by P. Waltereit, O. Brandt, A. Trampert, H.T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog,“Nitride semiconductors free of electrostatic fields for efficient whitelight-emitting diodes,” Nature 406, 865 (2000). The nonpolar planes ofGaN form a 90° angle with the c-plane and the semipolar planes form anintermediate angle between 0° and 90°. The semipolar planes also arethose planes with a nonzero h, k, or i index as well as a nonzero lindex in the Miller-Bravis indexing convention. Various crystallographicplanes of hexagonal GaN crystal material, including polar, nonpolar, andsemipolar planes, are shown in FIG. 1.

Unlike the basal c-plane in GaN, the nonpolar planes and semipolarplanes show an unequal photon emission that is dependent on thedirection of the electric vector, i.e.; the intensity of the lightperpendicular (E⊥c) and parallel (E∥c) to the c-axis is unequal. UsingS. L Chuang and C. S. Chang, “k-p method for strained wurtzitesemiconductors,” Physical Review B, 54, 2491-2504 (1996), who derivedthe effective-mass Hamiltonian for wurtzite semiconductors includingstrain, J. B. Jeon, B. C. Lee, Yu. M. Sirencko, K. W. Kim, and M. A.Littlejohn, “Strain effects on optical gain in wurtzite GaN,” Journal ofApplied Physics, 82, 386-391 (1997) was able to lay theoretical work onthe anisotropy in the optical gain of wurtzite GaN and showed that thisanisotropy is caused by the anisotropic valence bands. Mathematically,they showed that using the averaged momentum matrix element which wastaken from the full 6×6 Hamiltonian for the valence bands, the valenceband factors must contribute to the anisotropy since they are unequal.The valence bands for GaN are mainly composed of N 2p states and, at theBrillouin-zone center where the wave vector k=0 (Γ point), the valencebands with atomic p_(x) and p_(y) character are degenerate and p_(z)lies at a lower energy, thus splitting the valence band into two (knownas the crystal field splitting, Δ_(cr)) without considering thespin-orbital interaction. The spin-orbital interaction causes the stateswith p_(x) and p_(y) character to no longer be degenerate and forms theheavy hole (HH) and light hole (LH) bands for c-plane orientation. Theenergy band characteristics near the conduction band minimum don'tsuffer from this type of splitting since the states there are mainlycomposed of N and Ga s orbital character which are symmetrical in alldirections.

The wave functions (basis states) for the valence bands in the c-plane(taken in the x-y plane) are defined by S. L Chuang and C. S. Chang,“k-p method for strained wurtzite semiconductors,” Physical Review B,54, 2491-2504 (1996) as:

$\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. {\left. \left| U_{1} \right. \right\rangle = \left. {- \frac{1}{\sqrt{2}}} \middle| \left. \left( {X + {iY}} \right)\uparrow \right. \right.} \right\rangle,\left| U_{2} \right.} \right\rangle = {\frac{1}{\sqrt{2}}❘\left. \left( {X - {iY}} \right)\uparrow \right.}} \right\rangle,\left| U_{3} \right.} \right\rangle = {❘\left. Z\uparrow \right.}} \right\rangle,\left| U_{4} \right.} \right\rangle = \left. \frac{1}{\sqrt{2}} \middle| \left. \left( {X - {iY}} \right)\downarrow \right. \right.} \right\rangle,\left| U_{5} \right.} \right\rangle = \left. {- \frac{1}{\sqrt{2}}} \middle| \left. \left( {X + {iY}} \right)\downarrow \right. \right.} \right\rangle,\left| U_{6} \right.} \right\rangle = \left| Z\downarrow \right.} \right\rangle$

It is clear from the above equation that the HH and LH bands are amixture of (|X±iY

) and in the c-plane orientation with isotropic biaxial strain (wherethe strain components ε_(xx)=ε_(yy)), there is no optical polarizationanisotropy. However, for the nonpolar and semipolar crystalorientations, the strain is no longer isotropic. Using the m-plane as anexample, it has been shown by K. Domen, K. Horino, A. Kuramata, and T.Tanahashi, “Analysis of polarization anisotropy along the c axis in thephotoluminescence of wurtzite GaN,” Applied Physics Letters, 71,1996-1998 (1997) that the crystal field in GaN is strong enough to fixthe axis of the p functions along the c axis. The original mixed statesin the c-plane orientation are no longer maintained and the states nowbecome |X

-like and |Y

-like S. L Chuang and C. S. Chang, “k-p method for strained wurtzitesemiconductors,” Physical Review B, 54, 2491-2504 (1996). The growth ofInGaN/GaN wells, for example on m-plane with compressive, biaxial,anisotropic, strain the energy in the z-direction (c axis, |Z

state which will be referred to as E_(v3)) which is now raised higherthan the |Y

state, E_(v2). The |X

state, E_(v1), is also raised from the increased strain but it wasalready higher than the other two states so it maintains its relativeposition as the top-most energy valence band and the |Y

state is lowered due to tensile strain along the m axis. A schematic ofthis argument is illustrated in FIG. 2.

When growth of GaN is directed along the c axis (z-direction), theelectric field components (E_(x) and E_(y)) will always be perpendicularto the c axis and, as the electrons transition to the valence bandsradiatively, there is no difference in their state of polarization andthus the emitted light is uniformly unpolarized. However, when growth ofGaN is directed along a nonpolar plane or semipolar plane (take them-plane as an example again), the electric field component E_(z) is nowparallel to the c axis whereas E_(y) is perpendicular to the c axis andthe radiative transition probability for E⊥c is higher and prefers theE_(v1) (|X

state) as compared to E∥c which is lower and prefers the E_(v3) (|Z

state). Hence, two different states of polarization are now available inthe light emission. To physically quantify the amount of polarization ina sample, its polarization ratio, ρ, defined as,

$\rho = \frac{\left( {I_{\bot} - I_{||}} \right)}{\left( {I_{\bot} + I_{||}} \right)}$

where I_(⊥) (I_(∥)) is the intensity of the light with polarizationperpendicular (parallel) to the c axis and determines the limit of themaximum value for contrast ratios in display technologies. The degree ofpolarization will always deviate from unity due to the mixing of p_(z)valence band states due to quantum confinement (B. Rau, P. Waltereit, O.Brandt, M. Ramsteiner, K. H. Ploog, J. Puls, and F. Henneberger,“In-plane polarization anisotropy of the spontaneous emission of M-planeGaN/(Al,Ga)N quantum wells,” Applied Physics Letters, 77, 3343-3345(2000)). A more thorough discourse on this effect is found in Y. Zhao,R. M. Farrell, Y.-R. Wu, and J. Speck, “Valence band states andpolarized optical emission from nonpolar and semipolar III-nitridequantum well optoelectronic devices,” Japanese Journal of AppliedPhysics, 53, 100206 (2014).

The proliferation of display technologies in the early 21^(st) centuryhas led to wide commercialization of varying display products. One ofthe most prevalent display systems is the liquid crystal display(“LCD”). A common type of LCD is the twisted nematic liquid-crystaldisplay. It functions by having two electrode surfaces that providehomogeneous boundary conditions but with the two preferred orientationdirections being rotated by 90° with respect to each other. In theabsence of an electric field, a uniformly twisted region of nematicphase across the thickness of the device is achieved. When an electricfield is provided that is perpendicular to the thin liquid film, thedielectric anisotropy of the liquid-crystal molecules causes them toturn and to align with the field direction. When the field is turnedoff, the molecules revert back to their original state.

The image contrast in an LCD device is achieved by reflective lightutilizing an optical polarizer near the surface of both electrodes. Thebottom LCD substrate is mirrored on the underside for high reflectivity.Light that is unpolarized enters through the top of the device and ispolarized parallel to the upper orientation direction. If the electrodeis in the “off” state, then the light proceeds through the device andthe polarization follows the orientation of the liquid-crystal moleculesas they twist through 90°. Next, the light passes through the bottompolarizer to the reflecting surface, bounces back through the bottompolarizer, reverses orientation again through the liquid-crystalmolecules and passes through unhindered by the top polarizer. This “off”state therefore appears bright to the viewer since they are seeing theambient light that first entered the device. For the “on” state, thelight again enters the top polarizer but now the electrodes areactivated and the liquid-crystal molecules are aligned normal to thesubstrate. Therefore, no rotation of the polarization direction occursso no light passes through to the bottom polarizer to be reflected backto the viewer. In this case, the “on” state appears dark to theobserver. This “off” and “on” state produces acceptable image contrastfor the display.

Three of the leading small form factor electronic display structuresutilize reflective technology in one form or another; switchable mirrorsthat move into an on/off position (U.S. Pat. No. 5,083,857) and laserbeam steering with a scanning mirror (U.S. Pat. No. 6,245,590) eachrequire some type of a light source to be integrated into their MEMSdevice; liquid crystal on silicon (“LCoS”) incorporates a CMOSreflective layer (U.S. Pat. No. 7,396,130 and US Pub. No. 2004/0125283)each require a polarized light source for a complete system; and activematrix OLEDs and LEDs; while boasting less complexity, still benefitfrom an additional polarizer element to eliminate ghost images caused byreflections (U.S. Pat. Nos. 5,952,789 and 9,159,700).

Numerous prior art display devices utilize additional polarizingelements in order to create polarized light for use in displays.Superfluous objects that transform the electric field of the light afterit has been emitted from the active area of the device are a commonsolution. Examples of applications pertinent to the present inventioninclude, but are not limited to, external polarization layers,stand-alone polarization separation films with or without the use ofphase plates, periodic grating structures, and polarizing beamsplitters, etc. (U.S. Pat. Nos. 8,125,579, 6,960,010, 8,767,145,7,781,962, 7,325,957, 7,854,514 and US Pub. No. 2005/0088084).

Specifically, US Pub. No 2008/0054283 uses a plurality of metalnanowires as a polarization control layer that is grown as an additionalsemiconductor layer on the structure. One prior art example, WO2012140257, utilizes a semiconductor chip that emits polarized light byincorporating a lattice structure to selectively enhance the desiredradiation component, thus selecting which polarization state to emit.Nonetheless, this remains an additional element to be placed onto thesemiconductor chip in order for polarized light emission to occur andthe above prior art mentions nothing about how to actually incorporatethe polarization technology into a display product.

No prior art has been identified that describes a systematic method forachieving selective polarization states by utilizing variouscrystallographic planes of semiconductor materials for micro-LED displaypurposes.

A novel emissive imager is disclosed in U.S. Pat. Nos. 7,623,560,7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,098,265, 8,567,960 eachpatent of which is fully incorporated by reference herein, and is amicro-semiconductor emissive display device capable of efficientlygenerating the visible spectrum of electromagnetic radiation. Althoughthe usage of the disclosed emissive imager can be readily applied togeneral solid state lighting, the display applications of such a devicehave already been achieved. This emissive “Quantum Photonic Imager”(QPI) emissive display device emits photons from an active region of aIII-nitride semiconductor device and propagates the emitted light intofree space. “QPI” is a registered trademark of Ostendo Technologies,Inc., assignee of the present invention. In addition to QPI imagerswherein each pixel emits light from a stack of different color solidstate LEDs or laser emitters, imagers are also known that emit lightfrom different color solid state LEDs or laser emitters that aredisposed in a side by side arrangement with multiple solid state LEDs orlaser emitters serving a single pixel. Such devices of the presentinvention will be referred to generally as emissive display devices.Further, the present invention can be used to create light sources formany types of Spatial Light Modulators (SLMs, micro-displays) such asDLPs and LCOS and also can be used as a Backlight Source for LCDs aswell.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description, like reference are used for like elements,even in different drawings. The matters defined in the description, suchas detailed construction and elements, are provided to assist in acomprehensive understanding of the exemplary embodiments. However, thepresent invention can be practiced without those specifically definedmatters. Also, well-known functions or constructions are not describedin detail since they would obscure the invention with unnecessarydetail. In order to understand the invention and to see how it may becarried out in practice, a few embodiments of it will now be described,by way of non-limiting examples only, with reference to accompanyingdrawings, in which:

FIG. 1 depicts various crystallographic planes in a GaN crystalstructure.

FIG. 2 illustrates a coordinate system for a hexagonal crystal of GaN inwhich the z-axis is taken perpendicular to the c-plane, the y-axis istaken perpendicular to the m-plane, and the x-axis is takenperpendicular to the a-plane and the relative placement of the threevalence bands at the Γ point for the m-plane wherein E_(v2) has thelowest energy (not E_(v3)).

FIGS. 3A-3B illustrates views of the multicolor pixel comprising theemissive surface of a prior art Quantum Photonic Imager emissive displaydevice showing light emission from the c-plane or the <0001> direction.

FIGS. 4A-4C illustrate views of the multicolor pixel comprising theemissive surface of the Quantum Photonic Imager emissive display deviceof the invention showing light emission from the nonpolar m-plane or the<1010> direction in FIG. 4C.

FIGS. 5A-5D illustrate various multicolor pixel contact pads for use inthe polarized light emission Quantum Photonic Imager emissive displaydevice of the invention and a cross-section of a preferred embodiment ofthe invention showing X, Y and common contacts.

FIG. 6 illustrates a functional block diagram of the polarized lightemission Quantum Photonic Imager emissive display device of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above-referenced QPI emissive display device is but one example ofan emissive micro-scale pixel array U.S. Pat. Nos. 7,623,560, 7,767,479,7,829,902, 8,049,231, 8,243,770, 8,098,265, 8,567,960, referred to inthe exemplary embodiments described below. However, it is to beunderstood that the illustrated QPI emissive display device is merely anexample of the types of light emitting devices that may be used in andfabricated by the present invention, some of which have been previouslyset forth. Thus, in the description to follow, references to a QPIemissive display device are to be understood to be for purposes ofspecificity in the embodiments disclosed, and not for any limitation ofthe present invention U.S. Pat. No. 7,623,560.

FIG. 3A and FIG. 3B show the cross-section and light emitting surface ofa prior art QPI emissive display device or QPI imager as the c-plane andFIGS. 4A-4C show the side views and light emitting surface as them-plane in a preferred embodiment of the invention. The presentinvention enables emissive display devices such as the QPI emissivedisplay devices or QPI imagers and others to emit photons that eitheremit unpolarized or linearly polarized light.

FIGS. 4A-4C show a preferred embodiment of the invention as an exampleand not by way of limitation, and illustrate the QPI emissive displaydevice pixel structure comprising a stack of multiple solid state lightemitting layers comprising light emitting structures on top of asilicon-based semiconductor complementary metal oxide (Si-CMOS)structure comprising circuitry that is used to independently control theon-off state of each of the multiple solid state light emitting layersof the pixel structure. The surface dimensions of a QPI emissive displaydevice pixel are typically in the micro-scale with a pixel pitch rangingfrom 1 micron to 5 microns or larger. The QPI emissive display deviceitself may be comprised of a one or two-dimensional array of suchpixels, enabling user-desired pixel resolution in terms of the number ofrows and columns forming the array of QPI emissive display devicemicro-pixels.

The present invention satisfies a need in the display community forlight sources that can be tailored to emit various states of polarizedvisible light emission. Light weight, small form factor, and low powerconsumption are key considerations that manufacturers are cognizant ofwhen designing display systems for the consumer market, especially forhuman-wearable devices. Items such as glasses, goggles, wristbands,watches, or medical device monitors to name a few, all benefit frompolarized light displays that seamlessly integrate into a product andare not disruptive to the end user's application. To that end, thepresent invention allows the integration of a polarized light sourceinto a QPI emissive display device without the need for extraneoushardware that undesirably grows the display system or that requireshigher power input for the same number of output photons.

In preferred embodiments of the invention, a multicolor electronicemissive display device is provided comprising a two dimensional arrayof multicolor polarized light emitting pixel structures whereby eachmulticolor light emitting pixel comprises a plurality of light emittingstructures fabricated from a nonpolar or semipolar III nitride materialsystem. The light emitting structures may be configured each foremitting a different color of light and are each stacked vertically witha grid of vertical sidewalls electrically and optically separating eachmulticolor pixel from adjacent multicolor pixels within the array ofmulticolor pixels. A plurality of vertical waveguides are opticallycoupled to the light emitting structures to vertically emit polarizedlight generated by the light emitting structures from a first surface ofthe stack of light emitting structures. The stack of light emittingstructures are stacked onto a digital semiconductor structure on asecond surface opposite the first surface of the stack of light emittingstructures. A plurality of digital semiconductor circuits are providedin the digital semiconductor structure, each electrically coupled toreceive control signals from the periphery of the digital semiconductorstructure or from the bottom of the digital semiconductor structureusing plated through vias or plated interconnects on the sides of thedigital semiconductor structure, for example, for connection to theoutside world, so to speak. The plurality of digital semiconductorcircuits in the digital semiconductor structure are electrically coupledto the multicolor light emitting structures by vertical interconnectsembedded within the vertical sidewalls to separately control the on/offstates of each of the multicolor light emitting structures.

The prior art QPI emissive display device is a standalone emissivedisplay that eliminates the need for additional optical elements.Incorporated into such a device are layers of multicolor photonicelements, in this specific case, GaN or other III-V or II-VIsemiconductor materials that emit unpolarized, colored light. Thebackbone for pixel control logic is a digital semiconductor structurethat has been bonded together to form the QPI emissive display devicesystem. It may comprise digital drive logic circuits that provide powerand control signals to the stacked photonic semiconductor structure.This invention extends on the QPI device structures described in U.S.Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770,8,098,265, 8,567,960, or other micro-LED semiconductor arrays byincorporating a means of introducing intrinsic polarized light emissioncapabilities for various applications.

The photonic structure of the QPI emissive display device is comprisedof one or more individual layers of semiconductor material. In theinstance of a AlInGaN/GaN material system of the QPI emissive displaydevice, the c-plane is grown heteroepitaxially on a sapphire substrateby a growth techniques such as MBE, MOCVD, or HVPE. Other substratematerials may be used including, but not limited to, sapphire (Al₂O₃),hexagonal polymorphs of SiC, GaAs, Si, spinel (MgAl₂O₄), amorphoussilica (SiO₂), LiGaO₂, LiAlO₂, and ZnO. More recently, bulk GaNsubstrates show promising results for photonic devices but their higherprice and accessibility remain an obstacle (D. Ehrentraut, R. T.Pakalapati, D. S. Kamber, W. Jiang, D. W. Pocius, B. C. Downey, M.McLaurin, and M. D'Evelyn, “High quality, low cost ammonothermal bulkGaN substrates,” Japanese Journal of Applied Physics, 52, 08JA01 (2013)and W. Jie-Jun, W. Kun, Y. Tong-Jun, and Z. Guo-Yi, “GaN substrate andGaN homo-epitaxy for LEDs: Progress and challenges,” Japanese Journal ofApplied Physics, 24, 066105 (2015)) but can be incorporated as asubstrate material nevertheless. The substrate material of choice isaccompanied by a doped layer of AlN or GaN, followed by n-GaN:Si, then afixed number of InGaN/GaN multi-quantum wells (MQWs) with various indiumconcentrations depending on the desired emitted wavelength of the layer,then an AlGaN electron-blocking layer, and lastly the p-GaN:Mg. Thisphotonic structure is then patterned into a pixel array and contactsadded for forming the QPI emissive display device. In order to implementthe present invention, the growth direction of the QPI emissive displaydevice photonic layer (such as GaN/InGaN) is considered as is discussedbelow.

The orientation of the substrate used to grow GaN, specifically theatomic positions and surface chemistry, affects the dominant crystalplane that will coalesce during the epitaxial growth process. Beginningin the mid-1970s, studies were performed that identified the stablegrowth planes and preferred growth directions on various substrates andorientations for the GaN material system. Most of these studies focusedon sapphire since bulk Al₂O₃ crystals were readily available and hadalready been used for semiconductor epitaxy (P. A. Larssen,“Crystallographic match in epitaxy between silicon and sapphire,” ActaCrystallographica, 20, 599 (1966)). For example, c-plane and a-planesapphire substrates produce smooth c-plane GaN and AlN (H. M. Manasevit,F. M. Erdmann, and W. I. Simpson, “The use of metalorganics in thepreparation of semiconductor materials: IV. The nitrides of aluminum andgallium, Journal of the Electrochemical Society, 118, 1864 (1971)). Ithas been firmly established that the Ga-face c-plane wurtzite GaN is apreferred growth facet for planar thin film growth in this materialsystem. However, high quality nonpolar and semipolar films that utilizesubstrates such as (100) LiAlO₂ (P. Waltereit, O. Brandt, A. Trampert,H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog,“Nitride semiconductors free of electrostatic fields for efficient whitelight-emitting diodes,” Nature 406, 865 (2000)), r-plane Al₂O₃(M. D.Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, Structuralcharacterization of nonpolar (1120) a-plane GaN thin films grown on(1102) r-plane sapphire,” Applied Physics Letters, 81, 469 (2002)),6H—SiC (M. D. Craven, A. Chakraborty, B. Imer, F. Wu, S. Keller, U. K.Mishra, J. S. Speck, S. P. DenBaars, “Structural and electricalcharacterization of a-plane GaN grown on a-plane SiC,” Physica StatusSolidi (c) 0, 2132 (2003)), and more recently bulk m-plane and a-planeGaN gained traction when LED grown structures with comparable outputpowers to their c-plane counterparts were achieved (M. C. Schmidt, K.-C.Kim, H. Sato, N. Fellows, H. Masui, S. Nakamura, S. P. DenBaars, and J.S. Speck, “High power and high external efficiency m-plane InGaN lightemitting diodes,” Japanese Journal of Applied Physics, 46, L126 (2007)).Examples of such non-polar and semipolar films and applications aredisclosed in, for instance, U.S. Pat. Nos. 8,728,938, 9,443,727,8,629,065, 8,673,074, 9,023,673, 8,992,684, 9,306,116, 8,912,017,9,416,464, 8,647,435 and U.S. Pub. Nos. 2011/0188528 and 2014/0349427,each assigned to applicant Ostendo Technologies, Inc., assignee of theinstance application and the entirely of each of which is incorporatedherein by reference.

The present invention enhances the light output of photonic layers, suchas those found in a QPI emissive display device, by implementing growthof the semiconductor material on a substrate that favors a non-c-planeorientation of the AlInGaN material system. The present invention solvesthe limitation of the nature of the random polarization states of priorart emissive display devices by utilizing non-basal plane, polar orsemipolar GaN. In order to utilize this method in the fabrication of apolarized emissive micro-LED display such as the QPI emissive displaydevice, a novel pixelation process is disclosed to address the chemicalstructure and surface bonding molecules present due to different crystalorientations making the disclosed linearly polarized light source in aQPI emissive display device a disruptive technology over existingdevices. The illustrated photonic layer(s) of the QPI emissive displaydevice of the invention are grown in a fashion that is conducive tononpolar or semipolar plane growth and formation as the finalorientation. Substrates that can be used for growth in the nonpolar orsemipolar orientation include, but are not limited to, (100) LiAlO₂,r-plane Al₂O₃, m-plane Al₂O₃, hexagonal polymorphs of SiC, variousspinel ((100), (110), MgAl₂O₄) planes, (001) Si substrates with miscuts(7° for example), bulk m-plane, a-plane or a semipolar plane of GaN.Additionally, faceted sidewalls of lateral epitaxial overgrowth ofc-plane GaN can be used as a substrate for non-c-plane orientation.These various substrates will produce non-c-plane orientation in theIII-nitride material system and induce optical polarization anisotropyin the emitted light.

Another embodiment of the present invention is the introduction ofanisotropic strain in the c-plane of GaN. This also allows opticalpolarization to occur due to a compressive or tensile strain on the GaNepilayers.

Each of the layers within the stack of multiple solid state lightemitting layers comprising the exemplar QPI emissive display devicepixels (see FIGS. 4A-4C) are designed to emit a different colorwavelength, thus allowing the QPI pixel to be controlled through itsSi-CMOS to emit any desired combination of multiple colors; for example,red (R), green (G) and blue (B), from the same pixel aperture to coverany desired color gamut based on the selected color coordinates of theselected RGB emission wavelengths.

The fabrication process of a preferred embodiment of the polarized lightemission QPI emissive display device structure illustrated in FIGS.4A-4C includes the steps described in the following paragraphs. Theprocess begins with forming the QPI pixels array on the topside surfaceof a semiconductor light emitting photonic wafer, epitaxially grown toemit light from the nonpolar m-plane or the <1010> direction. Thisprocess, which herein is referred to as pixelation, involves the etchingof the pixel sidewalls, about 1 micron in width and depth, to extendthrough the heterojunction diode structure of the semiconductor lightemitting material, using semiconductor lithography and etch processes.The etched pixel array sidewalls are passivated with either a siliconoxide or silicon nitride thin layer using semiconductor depositionprocesses, then coated with a thin layer of reflective metal, such asAluminum (Al), for example. The pixel sidewalls are then filled withmetal, such as nickel, for example, using semiconductor metal depositionprocesses. After the pixelation pattern is processed on the topsidesurface of the polarized emission photonic wafer, alignment marks areadded on the wafer to aide alignment of the etched pixel pattern duringsubsequent processing.

The same topside surface pixelation process may be performed on multiplepolarized emission semiconductor light emitting photonic wafers, eachhaving a different wavelength polarized light emission, for example,465-nm (B), 525-nm (G) and 625-nm (R). These three topside surfaceprocessed polarized emission semiconductor light emitting photonicwafers may then be bonded one upon the other in a stack, as described inthe following paragraphs, to form a polarized light emission RGB QPIemissive display device.

After the topside surface of the polarized emission semiconductor lightemitting photonic wafers are pixelated, one of the topside contact metalpatterns illustrated in FIGS. 5A-5C is deposited on each of the formedpixel arrays using semiconductor metal deposition techniques such aselectron beam deposition. The contact metal pattern illustrated in FIG.5A may be used for the Blue (B) polarized light emission photonic waferand the contact metal pattern illustrated in FIG. 5B may be used for thetopside of the Green (G) and Red (R) polarized light emission photonicwafers. The deposited contact metal is preferably a thin metal stack,for example Ti/Al, that forms an ohmic contact with the indium galliumnitride (InGaN) heterojunction diode semiconductor light emittingstructure of the B, G and R polarized emission photonic wafers.

After the deposition of the contact layers, the topside of the B, G andR polarized light emission photonic wafers are processed further to formthe pixel sidewalls through the semiconductor epitaxial layers whichincludes etching the pixels' sidewalls, passivation, then metallizationand metal fill deposition. This step makes the pixel's sidewallselectrically conductive as well as optically blocking and reflective.These features of the pixel's sidewalls also prevent optical crosstalkbetween neighboring pixels, confine the generated light within theformed pixel reflective sidewall cavity and serve as an electricalinterconnect via to conduct electrical signals to the pixels' topsidecontacts as well as the pixels' contacts of the topside stacked photoniclayers.

In a preferred embodiment for fabricating the polarized light emissionQPI emissive display device structure illustrated in FIGS. 4A-4C, aglass wafer (not shown) may be used as a substrate upon which themulti-layer pixel array structures are stacked, then bonded to thetopside of a Si-CMOS wafer that is processed to include the same pixelcontact pattern as the multi-layer pixel array structure stacked on theglass wafer.

In an additional preferred embodiment for fabricating the polarizedlight emission QPI emissive display device structure, the Si-CMOS isused as a substrate upon which the multi-layer pixel array structuresare stacked, then the pixelated multi-layer wafer is bonded to a glasscover wafer. In either of the two above embodiments, the processingsteps are similar and the former will be used as an example, and not bylimitation, to describe the remaining steps of the polarized lightemission QPI display fabrication process.

FIGS. 5A-5C illustrate three different metal contact patterns used forthe polarized light emission QPI emissive display device micro-pixel'smetal contact layers that are deposited on the topside of the pixelatedB, G and R photonic wafer, using conventional semiconductors andlithography and metal deposition. The pixel contact pattern shown inFIG. 5A may be used on the topside of the pixelated B photonic wafers togenerate collimated (for example, ±17°) to quasi-Lambertian (forexample, ±45°) pixel's polarized light emission when the contactopening's diameter, height and spacing are selected to form auser-defined optical waveguide for extracting the light emitted from thepolarized light emission QPI pixels. The pixel contact pattern shown inFIG. 5B is used on the topside of the pixelated B photonic wafers togenerate Lambertian emission from the polarized light emission QPIpixels. The pixel contact pattern shown in FIG. 5B is also used on thepixelated G and R photonic wafers to allow maximum light transmissionfrom the lower to upper layers of the polarized light emission QPIpixels' structures.

A yet further preferred embodiment of the polarized light emission QPIemissive display device pixel structure is illustrated in FIG. 4Awherein a glass cover wafer is first processed to pattern an array ofpixel-size micro-optical elements or micro lenses that match thepolarized light emission QPI pixel array pattern. When the glass coverwafer with the pixel-sized micro-optical micro lens elements is used asthe substrate upon which the polarized light emission QPI multi-layerstack is formed, the resultant pixel array possesses the addedcapability, in addition to modulating the pixels array color andbrightness, of modulating the direction of the pixel's light emission; acapability that enables modulation of light field for direct view aswell as wearable near-eye displays.

After the B polarized light emission photonic wafer topside is pixelatedand its topside contact layer is deposited, the wafer is then bonded tothe glass cover wafer, with or without the pixel-size micro-opticalmicro lens elements incorporated, using semiconductor bonding techniquessuch as fusion bonding, for example. The epitaxial growth sapphire waferis then lifted off, using known semiconductor laser lift off (LLO)techniques and the structure is thinned down to remove the epitaxialgrowth GaN buffer leaving a thin layer (<2 micron) comprising the Bsemiconductor polarized light emitting heterojunction diode structureenclosed within the formed pixels' sidewalls. With the backside of thepixelated B polarized light emission photonic wafer exposed, the pixelarray backside contact pattern illustrated in FIG. 5B, is deposited asthin metal stacks, for example Ti/Al, using semiconductor metaldeposition techniques.

Herein the term “in-process QPI wafer” is used to refer to the processedmulti-layer stacked wafer which incorporate the multiple layers stackedup to that point the process. In using such terminology, the topside ofthe in-process QPI wafer thus, as illustrated, becomes the backside ofthe last layer bonded.

The topside of the in-process QPI wafer, which is the pixelated Bphotonic layer backside, is then processed to deposit the bondingintermediary layer incorporating electrical contact vias aligned withthe pixels' array sidewalls. The bonding intermediary layer is a thinlayer of either silicon oxide or silicon nitride deposited usingconventional semiconductor deposition techniques such as plasma-assistedchemical vapor deposition (PECVD), for example. After the bondingintermediary layer is deposited, the in-process QPI wafer surface isplanarized to a surface planarization level sufficient for bonding withthe topside of other wafers to form the multi-layer stack of thepolarized light emission QPI emissive display device.

The topside of the in-process QPI wafer, which would be the B layerbackside with the bonding intermediary layer added, is then processed tobond it to the G photonic wafer topside. This is accomplished usingaligned bonding of the pixelated G photonic wafer to the topside of theQPI in-process wafer using semiconductor bonding process such as fusionbonding, for example.

With the pixelated G photonic wafer bonded to the in-process QPI wafer,the epitaxial growth sapphire wafer of the G polarized light emissionphotonic wafer is lifted off, typically using semiconductor laser liftoff (LLO) techniques, and the structure is thinned down to remove theepitaxial growth GaN buffer leaving only a thin layer (<2 micron)comprising the G semiconductor polarized light emitting heterojunctiondiode structure enclosed within the formed pixel sidewalls. With thebackside of the pixelated G photonic wafer exposed, the pixel arraybackside contact pattern of FIG. 5B, is deposited of a thin metal stack,for example Ti/Al, using semiconductors metal deposition techniques.

The topside of the in-process NCP-QPI wafer, which is the pixelated Gphotonic layer backside, is then processed to deposit the bondingintermediary layer incorporating electrical contact vias aligned withthe pixels' array sidewalls. The bonding intermediary layer is a thinlayer of either silicon oxide or silicon nitride deposited usingconventional semiconductor deposition techniques such as plasma assistedchemical vapor deposition (PECVD), for example. After the bondingintermediary layer is deposited, the in-process QPI wafer surface isplanarized to a surface planarization level sufficient for bonding withtopside of other wafers to form the multi-layer stack of the polarizedlight emission QPI device.

The topside of the in-process QPI wafer, which would be the G layerbackside with the bonding intermediary layer added, is then processed tobond it to the R photonic wafer topside. This is accomplished usingaligned bonding of the pixelated R photonic wafer to the topside of theQPI in-process wafer using semiconductor bonding processes such asfusion bonding, for example.

With the pixelated R photonic wafer bonded to the in-process QPI wafer,the epitaxial growth sapphire wafer of the R polarized light emissionphotonic wafer is then lifted off, typically using semiconductor laserlift off (LLO) techniques, and the structure is thinned down to removethe epitaxial growth GaN buffer leaving only a thin layer (<2 micron)comprising the R semiconductor polarized light emitting heterojunctiondiode structure enclosed within the formed pixel sidewalls. With thebackside of the pixelated R photonic wafer exposed, the pixel arraybackside contact pattern of FIG. 5C, is deposited of a thin metal stack,for example Ti/Al, using semiconductor metal deposition techniques.

As illustrated in FIG. 5C, the topside of the QPI in-process wafer hasthree contact vias per pixel; the center contact via, which is theunique contact of the R photonic layer of the pixel, the x-sidewallcontact via which is the unique contact of the B photonic layer of thepixel and the y-sidewall contact via, which is the unique contact of theG photonic layer of the pixel. The common contacts for the entire pixelarray; namely, the three intermediate contact layers added on thetopside of the B, G and R photonic layers are formed as common contactrails that extend to the peripheral edges of the polarized lightemission QPI die where they are connected to a set of common contactvias forming a ring at the peripheral boundaries of each of thepolarized light emission QPI die comprising the in-process QPI wafer.

The in-process QPI wafer topside is then comprised of an array ofmicro-scale contact vias whereby the pixel-center via is the uniquecontact of the R photonic layer of the pixel array, the x-sidewallcontact via is the unique contact of the B excitation photonic layer ofthe pixel array, the y-sidewall contact via is the unique contact of theG emission photonic layer of the pixel array and the micro-via ring atthe peripheral boundaries of each of the QPI dies comprises thein-process QPI wafer providing the common contacts of all three photoniclayers of the pixel array comprising the QPI multi emissive layer stack.

As illustrated in FIG. 5D, the topside of each the QPI emissive displaydevice dies comprising the Si-CMOS wafer includes a micro-via array witha pattern matching the pattern of micro-via array of the in-process QPIwafer described in the previous paragraph. When the Si-CMOS wafer isaligned and bonded to the in-process QPI emissive display device waferusing semiconductor bonding techniques, such as fusion bonding, forexample, the bonding interface micro-via array provides electricalcontact between the unique contacts of the pixel arrays of the multiplephotonic layers of the polarized multi-color polarized light emissionQPI emissive display device as well as the common contact ring at theperipheral boundaries of each of the die comprising the QPI emissivedisplay device wafer.

It was previously mentioned that the introduction of anisotropic strainin the c-plane of GaN allows optical polarization to occur due to acompressive or tensile strain on the GaN epilayers. In particular, if atleast one of the layers is fabricated from a c-plane orientation GaNmaterial system of epilayers wherein an anisotropic strain is induced inthe c-plane GaN, an optical polarization in an emitted light from thelight emitting structures occurs due to a compressive or tensile strainon at least one of the GaN epilayers (strain components ε_(xx)≠ε_(yy)).One way of achieving this affect is to deliberately introduce straininto the GaN layers. Growth of GaN on A-plane sapphire causes suchstrain as well as Al-rich AlN/Al_(x)Ga_(1-x)N quantum wells orAl_(x)Ga_(1-x)N strain compensation layers. In this case Al-rich quantumwells, the strain compensation layers would be integral layers of thepolarized light emissive structure. Nanostructures grown on c-plane isanother way to realize polarized emission. Examples include photoniccrystals, metallic nanoparticles, elliptic nanorods and nano-gratings.Again, in the case of the elliptic nanorods and nano-gratings, thein-plane strain asymmetry is attributed for the creation of thepolarized light.

FIG. 6 illustrates a functional block diagram of the multi-colorpolarized light emission QPI emissive display device. FIG. 6 shows themulti-color micro-pixel array of the QPI emissive display device beingdriven by the control logic of its Si-COMS. FIG. 6 also shows twopossible embodiments for the QPI Si-CMOS control logic with two possibleinterfaces. In the first embodiment (above (A)), the function of the QPISi-CMOS control logic includes only the multi-color, micro-pixel arraydrivers and the QPI emissive display device in this case will receivecontrol signals and pixel array bit-fields containing the pulse widthmodulation (PWM) bits for every color of each pixel from an externalsource. In the second embodiment (above (B)), the function of the QPISi-CMOS control logic may additionally include the logic functionrequired to generate PWM bit-fields for the multi-color micro-pixelarray.

In the second embodiment, the QPI Si-CMOS control logic receives aserial bit-stream containing the light modulation video input andrelated control data through its interface block. In this embodiment ofthe polarized multi-color emission QPI emissive display device, theSi-CMOS control logic received light modulation video bit-stream isprocessed by the Color & Brightness Control block for de-gammalinearization, gamut transformation, white point adjustment and colorand brightness uniformity correction across the micro-pixel array. Thebit stream output of the Color & Brightness Control block is thenconverted to PWM bit-fields, then clocked into the pixel driver arrayincorporated together within the QPI Si-CMOS. In effect, the latterembodiment of the QPI Si-CMOS control logic of the polarized lightemission QPI emissive display device does not require external videostream processing support and operates with a standard high speedinterface such as Low Voltage Differential Signaling (LVDS) interface orthe like. The latter embodiment of the QPI Si-CMOS enables lower powerconsumption and smaller volumetric aspects for the polarized lightemission QPI applications. In either embodiment, the connections to theoutside world may be, for example, as already described herein.

One of the primary advantages of the described polarized light emissionQPI emissive display device is its low power consumption which isachieved by multiple factors: (1) the high internal quantum efficiency(IQE) of its photonic layers; (2) the high quantum yield (QY) conversionefficiency of the directly polarized multi-color light emission from itsemissive multi-layers; (3) the increased optical aperture conversionefficiency of its V-B excitation light by the light confinement actionsof the NPC-QPI pixel optical cavity; (4) the increased conversionefficiency of its V-B excitation light by the light confinement actionsof the optical sub-cavities formed by the pixel's BPF layers, andreflective sidewalls and contacts; and; (5) the spectral shaping actionsof the pixel's BPF layers to match the HVS photopic response.

The low power consumption of the described polarized light emission QPIemissive display device makes it very effective in display applicationsrequiring small volumetric aspects and higher brightness at low powerconsumption such as near-eye displays for virtual and augmented reality(AR/VR) applications. The wavelengths selected (only primary colors setforth) in preceding description of the multiple embodiments of thisdisclosure are for example purpose and other selections of thesewavelengths following the same methods of this invention are within thescope of this invention. Also, the emissive micro-scale pixels combinedwith the low power consumption of the described polarized light emissionQPI emissive display device make it very effective in light fielddisplay applications which typically require micro-scale pixel pitches,small volumetric aspects and higher brightness at low power consumptionplus require a directionally modulated micro-pixel. Of course, thecombination of these two display applications; namely, light fieldnear-eye AR/VR display, stands to benefit substantially by the smallvolume, high brightness, light field modulation and low powerconsumption capabilities of the polarized light emission QPI emissivedisplay device of this invention.

It is noted that the emission wavelength values used in the precedingdescription of the polarized QPI emissive display device structure ofthis invention are exemplary illustrations of the methods of thisinvention. A person skilled in the art of light emitting structureswould recognize how to use the disclosed methods of this invention tocreate a emissive micro-pixel spatial light modulator having polarizedlight emission using a different set of light wavelengths to generatedifferent sets of emission wavelengths. A person skilled in the artwould recognize how use disclosed methods of the polarized lightemission QPI emissive display device structure pixels' opticalconfinement created by the pixels' reflective sidewalls, reflectivecontacts and electrical interconnect sidewalls with different designparameters to create high efficiency multi-color micro-pixel arraydevice that emits polarized light.

It is further noted that the nonpolar and semipolar crystallineorientation of the polarized light emission QPI emissive display deviceenables higher intake ratios of indium in the epitaxial growth of theInGaN/GaN heterojunction diode structure of the semiconductor lightemitting material. Such higher indium intake ratios enable the emissionof polarized long wavelength light in the Amber (615-nm) to Red (625-nm)range with excellent IQE and saturation characteristics and thefabrication of efficient polarized light emission QPI emissive displaydevices having multi color emission covering the full span of thevisible light spectrum. This is an important advantage of the polarizedlight emission QPI emissive display device described in this disclosurebecause it is a known challenge to achieve similar results using polarcrystalline orientation because of indium's segregation at high intakeratios.

It is yet further noted that the methods for the fabrication of emissivemulti-color polarized light emission QPI display structures described inthis disclosure can be combined with the methods for the fabrication ofnon-polarized light emission QPI emissive display devices described inU.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770,8,098,265, 8,567,960 to enable the fabrication of multi-color emissionQPI emissive display devices with polarized and non-polarized lightemission at different emission wavelengths across the visible lightspectrum. Such light modulation capability enables a new class ofdisplays that benefit from the emission of both polarized andnon-polarized light emission from an emissive micro-pixel array atdifferent emission wavelengths across the visible light spectrum.

It is also important to note that the methods for the fabrication of thepolarized light emission QPI emissive display device described in thisdisclosure can be readily used to create a polarized light emission QPIemissive display device having a single wavelength emission by executingthe described fabrication process using only one photonic wafer havingthe desired emission wavelength.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention without departing from its scope defined in and by theappended claims. It should be appreciated that the foregoing examples ofthe invention are illustrative only, and that the invention can beembodied in other specific forms without departing from the spirit oressential characteristics thereof. The disclosed embodiments, therefore,should not be considered to be restrictive in any sense. The scope ofthe invention is indicated by the appended claims, rather than thepreceding description, and all variations which fall within the meaningand range of equivalents thereof are intended to be embraced therein.

What is claimed is:
 1. A multicolor electronic emissive display devicecomprising: a plurality of semiconductor layers, each stacked one uponthe other; each layer configured for emitting a different color ofvisible light and each layer comprising a two dimensional array ofvisible light emitting structures; and, wherein at least one of thelayers is fabricated from a c-plane orientation GaN material system ofepilayers wherein an anisotropic strain is induced in the c-plane GaN,whereby an optical polarization in an emitted visible light from thevisible light emitting structures occurs due to a compressive or tensilestrain on at least one of the GaN epilayers.
 2. The device of claim 1wherein an anisotropic strain is induced in the c-plane GaN byintroducing strain into at least one of the GaN epilayers.
 3. The deviceof claim 2 wherein an anisotropic strain is induced in the c-plane GaNby growth of the GaN epilayer on A-plane sapphire.
 4. The device ofclaim 2 wherein an anisotropic strain is induced in the c-plane GaN byusing Al-rich AlN/AlxGa1-xN quantum wells or AlxGa1-xN straincompensation layers.
 5. The device of claim 2 wherein an anisotropicstrain is induced in the c-plane GaN by nanostructures grown on c-planeGaN.
 6. The device of claim 5 wherein the nanostructures include atleast one of photonic crystals, metallic nanoparticles, ellipticnanorods or nano-gratings.